linux-kernel - Re: [PATCH v6 2/2] rust: add dma coherent allocator abstraction.

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Message-ID: <c9baa4bb-81db-42ae-98be-d8cb7d57f237@gmail.com>
Date: Thu, 5 Dec 2024 16:25:53 +0200
From: Abdiel Janulgue <abdiel.janulgue@...il.com>
To: Robin Murphy <robin.murphy@....com>, daniel.almeida@...labora.com
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
 Valentin Obst <kernel@...entinobst.de>,
 "open list:RUST" <rust-for-linux@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com,
 "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v6 2/2] rust: add dma coherent allocator abstraction.

On 05/12/2024 15:07, Robin Murphy wrote:
>> +    pub fn alloc_coherent(
>> +        dev: &Device,
>> +        count: usize,
>> +        flags: kernel::alloc::Flags,
> 
> Might it be worth adding at least a placeholder type and argument for 
> "attrs" as well, so there's less churn involved in coming back and 
> adding it later? Or is the intention to have a separate 
> dma_alloc_attrs() constructor for such users?
> 

It certainly is possible to include attrs in the constructor using Option:
```
     pub fn alloc_coherent(
         dev: &Device,
         count: usize,
         flags: kernel::alloc::Flags,
         attrs: Option<u64>,
     ) -> Result<CoherentAllocation<T>> {

<cut>
         let ret = unsafe {
             bindings::dma_alloc_attrs(
                 dev.as_raw(),
                 size,
                 &mut dma_handle, flags.as_raw(),
                 attrs.unwrap_or(0),
             )
         };
<cut>
     }

```
So the signature of the constructor could be:

let c = CoherentAllocation::alloc_coherent(dev, 4, GFP_KERNEL, None)?;

Andreas, Miguel, Daniel, any thoughts?

Regards,
Abdiel